Exhaust emission control filter and method of controlling exhaust emission

ABSTRACT

Carbon-containing particulates like soot included in an exhaust gas of an internal combustion engine are collected by means of a heat-resistant filter medium. The filter medium dispersedly collects hydrocarbon compounds and the carbon-containing particulates included in the exhaust gas. Even when the exhaust gas has a lower temperature than the combustible temperature of the carbon-containing particulates, such dispersed collection ensures start of a preliminary oxidation reaction of the collected hydrocarbon compounds with oxygen included in the exhaust gas. The collected hydrocarbon compounds and the collected carbon-containing particulates are then subjected to combustion with the exhaust gas having a filter inflow temperature lower than the combustible temperature of the carbon-containing particulates by utilizing the reaction heat and active species produced by the reaction. This technique thus simply but securely regulates and reduces the carbon-containing particulates included in the exhaust gas.

TECHNICAL FIELD

The present invention relates to a technique of regulating and reducingcarbon-containing particulates included in exhaust gases from aninternal combustion engine.

BACKGROUND ART

The exhaust gas from Diesel engines includes carbon-containingparticulates like black smoke (soot), and there is a high demand ofreducing the total emission of the carbon-containing particulates, inorder to prevent further air pollution. There is a similar demand indirect injection gasoline engines where gasoline is directly injectedinto a combustion chamber, since the carbon-containing particulates maybe discharged with the exhaust gas under some driving conditions.

One proposed technique of remarkably reducing the carbon-containingparticulates in the emission from an internal combustion engine disposesa heat-resistant filter in an exhaust conduit of the internal combustionengine and uses the filter to collect the carbon-containing particulatesincluded in the exhaust gas.

The particulates are mainly composed of carbon, but should be exposed tohigh temperatures of not lower than 550° C. in the exhaust gascontaining oxygen for their combustion. As long as the internalcombustion engine, such as the Diesel engine or the direct injectiongasoline engine, is driven under ordinary conditions, the temperature ofthe exhaust gas flown into the filter hardly exceeds the temperature of550° C. It is accordingly required to process the collected particulatesby some technique. Otherwise the filter is clogged to cause varioustroubles, for example, a decrease in output of the internal combustionengine.

A diversity of techniques have been proposed to process thecarbon-containing particulates collected by the filter. One simpletechnique makes a noble metal catalyst, such as platinum, carried on thefilter and utilizes the catalytic action for combustion of the collectedparticulates in the exhaust gas of a relatively low temperature (seeJAPANESE PATENT PUBLICATION GAZETTE No. 7-106290). Another proposedtechnique intentionally raises the temperature of the exhaust gas forcombustion of the collected carbon-containing particulates on the filter(see JAPANESE PATENT LAID-OPEN GAZETTE No. 2000-161044). There are avariety of methods applicable to raise the temperature of the exhaustgas. One applicable method called the intake reduction technique sets anon-off valve in an intake conduit of the internal combustion engine andnarrows the opening of the valve to increase the temperature of theexhaust gas. Another applicable method delays the injection timing ofthe fuel from the appropriate timing to increase the temperature of theexhaust gas.

These prior art techniques, however, have some drawbacks discussedbelow. The catalyst in use naturally deteriorates its performance. Thelong-term use of the catalyst thus makes it difficult to completelyprocess the collected carbon-containing particulates and eventuallyleads to clogging of the filter. Increasing the load of the noble metalon the filter certainly prevents the significant deterioration of thecatalyst, but it is not desirable to increase the load of the preciousnoble metal.

The technique of intentionally raising the temperature of the exhaustgas causes the chemical energy of the fuel not to be converted to theoutput of the internal combustion engine but to be released as heat.This undesirably lowers the output of the internal combustion engine orthe fuel consumption rate.

The object of the present invention is thus to solve the drawbacks ofthe prior art techniques discussed above and to provide a technique ofeasily and securely regulating and reducing carbon-containingparticulates included in the exhaust gas of an internal combustionengine over a long time period without worsening the performances of theinternal combustion engine.

DISCLOSURE OF THE INVENTION

At least part of the above and other related objects is attained by anemission filter of the present invention for regulating and reducingcarbon-containing particulates included in an exhaust gas from aninternal combustion engine. The emission filter has a heat-resistantfilter medium that collects hydrocarbon compounds and thecarbon-containing particulates included in the exhaust gas in adispersive manner to bring the respective particulates and hydrocarboncompounds in contact with oxygen included in the exhaust gas, andthereby makes the collected hydrocarbon compounds and the collectedcarbon-containing particulates subjected to combustion with the exhaustgas having a filter inflow temperature lower than a combustibletemperature of the carbon-containing particulates.

There is an emission control method corresponding to the emission filterdiscussed above.

The present invention is accordingly directed to an emission controlmethod that regulates and reduces carbon-containing particulatesincluded in an exhaust gas of an internal combustion engine. Theemission control method includes the steps of: utilizing aheat-resistant filter medium to collect hydrocarbon compounds and thecarbon-containing particulates included in the exhaust gas in adispersive manner to bring the respective particulates and hydrocarboncompounds in contact with oxygen included in the exhaust gas; and makingthe collected hydrocarbon compounds and the collected carbon-containingparticulates subjected to combustion with the exhaust gas having afilter inflow temperature lower than a combustible temperature of thecarbon-containing particulates, so as to regulate and reduce thecarbon-containing particulates.

In the emission filter and the corresponding emission control method ofthe present invention, the carbon-containing particulates included inthe exhaust gas from the internal combustion engine are collected by theheat-resistant filter medium, together with the hydrocarbon compounds inthe exhaust gas. The carbon-containing particulates represent anyparticulates containing carbon, such as soot. The hydrocarbon compoundsare non-combusted organic compounds attributed to the fuel or lubricantoil. The carbon-containing particulates and the hydrocarbon compoundsare dispersedly collected in the filter medium. A large portion of thecollected particulates and hydrocarbon compounds is kept in a specificstate that brings the respective particulates and hydrocarbon compoundsin contact with oxygen included in the exhaust gas. Even when thetemperature of the exhaust gas flown into the filter is lower than thecombustible temperature of the carbon-containing particulates, a gentleexothermic reaction proceeds between the collected hydrocarbon compoundsand oxygen. This eventually leads to combustion of the collectedhydrocarbon compounds and the collected carbon-containing particulates.This technique thus securely and easily reduces the carbon-containingparticulates included in the exhaust gas over a long time period.

The present invention has been completed, based on the findings ofunique phenomena. In order to clearly explain the functions and theeffects of the present invention, the unique phenomena found by theinventors are discussed briefly.

FIG. 27 conceptually shows the unique phenomena found by the inventors.FIG. 27(a) conceptually illustrates an experimental apparatus, where afilter E is disposed in an exhaust conduit of an internal combustionengine A (typically a Diesel engine). The internal combustion engine Aintakes the air from an intake conduit B, makes a fuel subjected tocombustion inside a combustion chamber C, and discharges the exhaust gasthrough an exhaust conduit D. The exhaust gas includes carbon-containingparticulates like soot and hydrocarbon compounds, which are collected bythe filter E disposed in the exhaust conduit D. The filter E is capableof dispersedly collecting the carbon-containing particulates and thehydrocarbon compounds as discussed later in detail. Measurable factorsare a temperature Tg of the exhaust gas flown into the filter E, atemperature Tf of the filter E, and a differential pressure ΔP beforeand after the filter E.

The graph of FIG. 27(b) shows variations in differential pressure ΔPbefore and after the filter, in temperature Tg of the exhaust gas in theupstream of the filter, and in filter temperature Tf when a newreplacement of the filter E is set in the exhaust conduit D and theinternal combustion engine A is driven in fixed conditions. In responseto a start of the operation of the internal combustion engine A, thetemperature Tg of the exhaust gas and the filter temperature Tfimmediately rise from room temperature to a steady temperature. In theactual state, the filter temperature Tf is higher than the temperatureTg of the exhaust gas. One of the unique phenomena found by theinventors is that the filter temperature Tf becomes higher than thetemperature Tg of the exhaust gas when the exhaust gas passes throughthe filter. This phenomenon will be discussed in detail later. Forsimplicity of explanation, here it is assumed that there is nosignificant difference between the filter temperature Tf and thetemperature Tg of the exhaust gas. The steady temperature, which thetemperature Tg of the exhaust gas and the filter temperature Tf reachesafter the start of the operation of the internal combustion engine A, isaffected by the driving conditions of the internal combustion engine Aand a diversity of other factors. The steady temperature is typically ina range of 250° C. to 350° C.

The differential pressure before and after the filter graduallyincreases even after the temperature reaches the steady state, but soonbecomes plateau and is substantially stabilized as shown in FIG. 27(b).The value of the stabilized differential pressure is varied mainly bythe design dimensions of the filter, but is typically three to fourtimes of the initial differential pressure. For convenience ofexplanation, the term from the start of the operation of the internalcombustion engine to the stabilization of the differential pressurebefore and after the filter is called the ‘first term’.

When the internal combustion engine continues driving afterstabilization of the differential pressure before and after the filter,the filter temperature Tf starts a gentle rise, whereas the temperatureTg of the exhaust gas flown into the filter is not significantly varied.With continuance of the operation of the internal combustion engine, thedeviation of the filter temperature Tf from the temperature Tg of theexhaust gas gradually increases. The filter temperature Tf eventuallyreaches about 550° C. The differential pressure ΔP before and after thefilter tends to slightly increase, due to collection of thecarbon-containing particulates like soot and the hydrocarbon compoundsby the filter E, although the level of increase may be insignificant.

When the filter temperature Tf rises to 550° C., the soot and the otherparticulates collected by the filter E starts combustion. The filtertemperature Tf once exceeds 550° C. but is soon lowered to a temperatureclose to the temperature Tg of the exhaust gas. This suggests thatcombustion of soot should be completed in a relatively short time. Inthe case where the increase in differential pressure ΔP before and afterthe filter, due to collection of soot and the other particulates in theexhaust gas, is detectable, a decrease in differential pressure ΔP, dueto combustion of the soot and the other particulates collected by thefilter E, is also detectable. For convenience of explanation, the termsubsequent to the first term when the filter temperature Tf graduallybecomes apart from the temperature Tg of the exhaust gas and is againdropped to the temperature Tg of the exhaust gas is called the ‘secondterm’. The first term is appreciably shorter than the second term. Forclarity of illustration, the first term illustrated in FIG. 27 is longerthan the actual length relative to the second term.

The filter temperature Tf is lowered to the temperature Tg of theexhaust gas on completion of combustion of the soot and the otherparticulates collected by the filter E, but again rises to 550° C. tostart combustion of the collected soot. Namely the filter E is kept inthe state of the second term to repeat collection and combustion of thesoot and the other particulates included in the exhaust gas.

The graph of FIG. 27(c) shows variations in filter temperature Tf and indifferential pressure ΔP before and after the filter when the drivingconditions of the internal combustion engine A are changed from theconditions of FIG. 27(b) to slightly raise the temperature of theexhaust gas flown into the filter E (typically by about 50° C.). Similarresults are obtained when the driving conditions are changed from theconditions of FIG. 27(b) to slightly raise the density of the soot orthe density of the hydrocarbon compounds, instead of the temperature ofthe exhaust gas.

As shown in FIG. 27(c), in the case of the little higher temperature ofthe exhaust gas flown into the filter E, the filter temperature Tf alsogradually becomes apart from the temperature Tg of the exhaust gas inthe second term and eventually reaches 550° C. to start combustion ofthe collected soot. Under the conditions of FIG. 27(c), since thetemperature Tg of the exhaust gas flown into the filter E is a littlehigher than that under the conditions of FIG. 27(b), the filtertemperature Tf reaches 550° C. in a shorter time period. Combustion ofthe collected soot is completed in a relatively short time and thefilter temperature Tf starts decreasing under the conditions of FIG.27(c). Unlike the case of FIG. 27(b), however, the decreasing filtertemperature Tf becomes plateau at temperature higher than thetemperature Tg of the exhaust gas. The term subsequent to the secondterm when the filter temperature Tf becomes plateau at the temperaturehigher than the temperature Tg of the exhaust gas is called the ‘thirdterm’. It is thought that the temperature difference between thetemperature Tg of the exhaust gas and the filter temperature Tf in thethird term depends upon the driving conditions of the internalcombustion engine A. The phenomenon occurring in the third term has notyet been elucidated, but it is expected that collection and combustionof the soot and the other particulates are locally repeated or thatcollection and combustion simultaneously proceed at an identical place.Anyhow the differential pressure ΔP before and after the filter is keptat a substantially constant value in the third term, as shown in FIG.27(c).

As described above, the inventors of the present application have foundthe phenomenon that dispersed collection of the carbon-containingparticulates and the hydrocarbon compounds included in the exhaust gasfrom the internal combustion engine to bring the respective particulatesand hydrocarbon compounds in contact with oxygen in the exhaust gasensures combustion of the collected carbon-containing particulates withthe exhaust gas having the lower filter inflow temperature than thecombustible temperature of the carbon-containing particulates. Thedetails of the test and the estimated mechanism of utilizing thelow-temperature exhaust gas for combustion of the carbon-containingparticulates will be discussed later.

The emission filter of the present invention and the correspondingemission control method utilize this phenomenon for combustion of thecarbon-containing particulates collected by the filter medium. Unlikethe prior art method of utilizing a catalyst for combustion of thecollected carbon-containing particulates and the method of intentionallyraising the temperature of the exhaust gas for combustion of thecarbon-containing particulates, the technique of the present inventionsecurely and readily regulates and reduces the carbon-containingparticulates in the exhaust gas without making the filter clogged ordeteriorating the performances of the engine. The specific function ofthe emission filter of the present invention that utilizes the abovephenomenon for combustion of the collected carbon-containingparticulates may be referred to as the ‘spontaneous regeneratingfunction’ in the specification hereof.

In accordance with one preferable application, heat of reaction of thehydrocarbon compounds collected by the heat-resistant filter medium withoxygen included in the exhaust gas is utilized to make the collectedcarbon-containing particulates subjected to combustion. The hydrocarboncompounds react with oxygen even in the exhaust gas of low temperaturethat does not cause combustion of the carbon-containing particulates.The arrangement of utilizing the heat of reaction of the hydrocarboncompounds with oxygen to raise the temperature of the heat-resistantfilter medium accordingly ensures combustion of the collectedcarbon-containing particulates with the exhaust gas having the lowerfilter inflow temperature than the combustible temperature of thecarbon-containing particulates.

In the emission filter of the above application, it is preferable thatthe heat-resistant filter medium utilizes active species produced by thereaction of the collected hydrocarbon compounds with oxygen included inthe exhaust gas, in addition to the heat of the reaction, so as to makethe collected carbon-containing particulates subjected to combustion.The presence of such active species generally tends to accelerate theoxidation reaction. The temperature of the heat-resistant filter mediumis raised by the heat of the reaction of the collected hydrocarboncompounds with oxygen. The active species produced through this reactionis further utilized to ensure combustion of the collectedcarbon-containing particulates.

In the emission filter, the heat-resistant filter medium may trap thecarbon-containing particulates and the hydrocarbon compounds therein. Itis rather difficult to dispersedly collect the carbon-containingparticulates and the hydrocarbon compounds mainly on the surface of thefilter medium. Trapping the carbon-containing particulates and thehydrocarbon compounds inside the filter medium, on the other hand,facilitates dispersed collection thereof.

In the emission filter that traps the carbon-containing particulates andthe hydrocarbon compounds therein, it is preferable that theheat-resistant filter medium utilizes a variation in pressure of theexhaust gas from the internal combustion engine to dispersedly collectthe carbon-containing particulates and the hydrocarbon compounds.Application of the pressure variation of the exhaust gas to thecarbon-containing particulates and the hydrocarbon compounds enables thecarbon-containing particulates and the hydrocarbon compounds to bereadily dispersed in and collected by the heat-resistant filter medium.

In accordance with another preferable application of the emissionfilter, the heat-resistant filter medium converts fluidization energy ofthe exhaust gas from the internal combustion engine into heat, so as toraise the own temperature of the heat-resistant filter medium. Theincreased temperature of the filter medium preferably facilitatescombustion of the collected carbon-containing particulates and thecollected hydrocarbon compounds even when the temperature of the exhaustgas flown into the filter is lower than the combustible temperature ofthe carbon-containing particulates.

In the emission filter of the above application, it is preferable thatthe heat-resistant filter medium is heated by utilizing a temperaturerise in the process of compressing the exhaust gas by means of a dynamicpressure. The dynamic pressure of the exhaust gas is effectivelyutilized to readily raise the temperature of the heat-resistant filtermedium and thereby ensure combustion of the collected carbon-containingparticulates.

In accordance with still another preferable application of the emissionfilter, the heat-resistant filter medium has multiple pathways, whichconnect with one another in a three dimensional manner inside the filtermedium and are open to surface of the filter medium. The heat-resistantfilter medium having such multiple pathways preferably enables thecarbon-containing particulates and the hydrocarbon compounds in theexhaust gas to be dispersed in and collected by the filter medium.

The multiple pathways formed inside the heat-resistant filter medium mayhave a mean inner diameter in a range of about 11 μm to about 13 μm.When the multiple pathways formed inside the filter medium have the meaninner diameter of less than 11 μm, the surface of the filter medium onthe inflow side of the exhaust gas is often clogged. The mean innerdiameter of greater than 13 μm, on the other hand, often causes thesurface of the filter medium on the outflow side of the exhaust gas tobe clogged. Setting the mean inner diameter of the multiple pathwaysformed inside the filter medium in the range of about 11 μm to about 13μm enables the carbon-containing particulates and the hydrocarboncompounds in the exhaust gas to be dispersed in and collected by thefilter medium without making the filter medium clogged. In thespecification hereof, the mean inner diameter represents the mean porediameter measured according to the Washburn's Equation. Namely the meaninner diameter is the pore diameter having the accumulated pore volumeof 50%. The numerical value of the mean inner diameter is varied inmeasurement of another known method.

A non-woven fabric made of heat-resistant fibers having a mean fiberdiameter in a range of about 15 μm to about 20 μm is applicable for theheat-resistant filter medium. It has been empirically found that themean inner diameter of pathways formed inside the non-woven fabric issomewhat correlated with the mean diameter of the fibers consisting ofthe non-woven fabric. The fibers having the mean diameter of about 15 μmto about 20 μm readily give the non-woven fabric having the mean innerdiameter of about 11 μm to about 13 μm. The fiber density (the number offibers per unit volume of the non-woven fabric) tends to be lowered withan increase in mean inner diameter of the pathways formed inside thenon-woven fabric. In order to compensate for a decrease in strength ofthe non-woven fabric due to the lowered fiber density, theheat-resistant fibers having the mean fiber diameter in the range ofabout 15 μm to about 20 μm are preferably used to make the non-wovenfabric having the mean inner diameter in the range of about 11 μm toabout 13 μm.

In the case where the non-woven fabric is applied for the heat-resistantfilter medium, the non-woven fabric preferably has a thickness in arange of about 0.3 mm to about 1.0 mm or more preferably has a thicknessin a range of about 0.4 mm to about 0.5 mm. The thinner non-woven fabricdoes not have sufficient strength and is easily broken. The excessivelythick non-woven fabric is, on the other hand, not readily bent and thusmakes it difficult to manufacture a relatively compact emission filter.Application of the non-woven fabric having the thickness of about 0.3 mmto about 1.0 mm or more preferably the thickness of about 0.4 mm toabout 0.5 mm for the heat-resistant filter medium facilitatesmanufacture of a relatively compact emission filter, while ensuring thepractically sufficient strength of the filter medium.

In the emission filter using the filter medium including the multiplepathways that connect with one another in the three dimensional manner,the heat-resistant filter medium may change over a flow path of theexhaust gas flowing through the multiple pathways in the course ofcollecting the carbon-containing particulates and the hydrocarboncompounds. The structure of changing over the flow path of the exhaustgas enables the carbon-containing particulates and the hydrocarboncompounds to be collected dispersedly in the heat-resistant filtermedium. Change-over the flow of the exhaust gas to a new flow path inthe course of collection preferably suppresses an increase in pressureloss when the exhaust gas passes through the filter medium.

In the emission filter of the above structure, the heat-resistant filtermedium may change over the flow path of the exhaust gas flowing throughthe multiple pathways when a pressure loss in the course of thecollection reaches three to four times of an initial value. After thepressure loss increases to three or four times of the initial value dueto collection of the carbon-containing particulates and the hydrocarboncompounds in the exhaust gas by the filter medium, the rate of increasein pressure loss tends to be heightened. This arrangement, which changesover the flow path of the exhaust gas passing through the filter mediumwhen the pressure loss reaches the three or four times of the initialvalue, desirably suppresses the increase in pressure loss.

In one preferable application, the emission control method discussedabove further includes the step of leading a supply of oxygen from theupstream of the heat-resistant filter medium into the exhaust gas. Thesupply of oxygen to the exhaust gas desirably accelerates the reactionof the hydrocarbon compounds collected on the filter medium with oxygenor accelerates the combustion of the collected carbon-containingparticulates with oxygen.

In another preferable application, the emission control method furtherincludes the step of arranging a NOx reduction catalyst in downstream ofthe heat-resistant filter medium to diminish nitrogen oxides included inthe exhaust gas. This arrangement desirably reduces both thecarbon-containing particulates and the nitrogen oxides included in theexhaust gas.

In the emission control method of the above application, the NOxreduction catalyst may be a catalyst that absorbs the nitrogen oxidesunder a condition that excess oxygen is present in the exhaust gas, andreduces the absorbed nitrogen oxides with a decrease in concentration ofoxygen in the exhaust gas. The NOx reduction catalyst that once absorbsand then reduces the nitrogen oxides included in the exhaust gasefficiently diminishes the nitrogen oxides in the exhaust gas. Thisarrangement thus efficiently reduces both the carbon-containingparticulates and the nitrogen oxides included in the exhaust gas.

The technique of the present invention is not restricted to the emissionfilter, but is attained by an emission control device with the emissionfilter discussed above.

The present invention is accordingly directed to an emission controldevice that is applied to an internal combustion engine to regulate andreduce carbon-containing particulates included in an exhaust gas. Herethe internal combustion engine has a combustion chamber and an exhaustconduit for discharging the exhaust gas from the combustion chamber. Theemission control device includes: an emission filter that is attached tothe exhaust conduit to collect the carbon-containing particulatesincluded in the exhaust gas; and a heat insulating section that isinterposed between the emission filter and the exhaust conduit. Theemission filter has a heat-resistant filter medium that collectshydrocarbon compounds and the carbon-containing particulates included inthe exhaust gas in a dispersive manner to bring the respectiveparticulates and hydrocarbon compounds in contact with oxygen includedin the exhaust gas, and thereby makes the collected hydrocarboncompounds and the collected carbon-containing particulates subjected tocombustion with the exhaust gas having a lower filter inflow temperaturethan a combustible temperature of the carbon-containing particulates.

There is also an emission control method corresponding to the emissioncontrol device.

The present invention is thus directed to an emission control methodthat is applied to an internal combustion engine to regulate and reducecarbon-containing particulates included in an exhaust gas. Here theinternal combustion engine has a combustion chamber and an exhaustconduit for discharging the exhaust gas from the combustion chamber. Theemission control method includes the steps of: disposing an emissionfilter, which has a heat-resistant filter medium, in the exhaust conduitin such a manner that a heat insulating section is formed between theemission filter and the exhaust conduit; utilizing the heat-resistantfilter medium to collect hydrocarbon compounds and the carbon-containingparticulates included in the exhaust gas in a dispersive manner to bringthe respective particulates and hydrocarbon compounds in contact withoxygen included in the exhaust gas; and making the collected hydrocarboncompounds and the collected carbon-containing particulates subjected tocombustion with the exhaust gas having a lower filter inflow temperaturethan a combustible temperature of the carbon-containing particulates, soas to regulate and reduce the carbon-containing particulates.

In the emission control device and the corresponding emission controlmethod of the present invention, the filter is disposed in the exhaustconduit to dispersedly collect the carbon-containing particulates andthe hydrocarbon compounds included in the exhaust gas. The heatinsulating section is formed between the emission filter and the exhaustconduit. As described later, the inventors of the present applicationhave newly found that the temperature of the emission filter rises whenthe flow of the exhaust gas passes through the filter. Formation of theheat insulating section between the emission filter and the exhaustconduit to prevent a release of heat to the exhaust conduit thusefficiently raises the temperature of the emission filter. The efficientrise in filter temperature ensures easy and secure combustion of thecarbon-containing particulates and the hydrocarbon compounds collectedon the filter.

The following describes the phenomenon that the filter temperature riseswhen the flow of the exhaust gas passes through the emission filter,with reference to FIG. 28. FIG. 28(a) conceptually illustrates anexperimental apparatus. As in the case of FIG. 27, a filter E isdisposed in an exhaust conduit of an internal combustion engine A(typically a Diesel engine). Measurable factors are the temperature Tgof the exhaust gas flown into the filter E and the filter temperatureTf.

The temperature Tg of the exhaust gas flown into the filter and thefilter temperature Tf were measured with this experimental apparatus,while the driving conditions of the internal combustion engine A werevaried. The measurement results showed that the filter temperature Tfwas always higher than the temperature Tg of the exhaust gas. Thetemperature Tg of the inflow exhaust gas and an increase dT (=Tf−Tg) ofthe filter temperature Tf were measured by varying the temperature ofthe exhaust gas, while the other factors like the flow rate of theexhaust gas were practically fixed. FIG. 28(b) shows the results of themeasurement.

As shown in FIG. 28(b), the increase dT of the filter temperature tendsto be linearly heightened with a rise in temperature Tg of the inflowexhaust gas. Based on this result, the following mechanism is estimatedfor the phenomenon of making the filter temperature Tf higher than thetemperature Tg of the inflow exhaust gas.

The filter E has a flow resistance and interferes with the flow of theexhaust gas having a large flow velocity, so that part of the flowvelocity of the exhaust gas is converted into a pressure. This gives apressure increase dP. According to the teach of thermodynamics, threevariables, that is, pressure P, temperature T, and specific volume valways satisfy the following relation:P·v=R·T  (1)where R denotes the gas constant. When the flow of the exhaust gas isintercepted by the filter E to increase the pressure P by dP, thetemperature of the exhaust gas increases by dT to satisfy Equation (1)given above. Namely this may be the mechanism of the phenomenon that thefilter temperature Tf is always higher than the temperature Tg of theexhaust gas. The dynamic pressure works to make the filter compress theexhaust gas and thereby raise the temperature of the exhaust gas. Thefilter is heated with the exhaust gas of the raised temperature. Thefilter temperature Tf is thus kept higher than the temperature Tg of theinflow exhaust gas.

The validity of this estimated mechanism is confirmed, based on themeasurement results shown in FIG. 28(b). Equation (1) is rewritten to:Pg·v=R·Tg  (2)where Pg denotes the pressure of the exhaust gas at the inlet of thefilter E and Tg denotes the temperature of the exhaust gas at the inlet.When it is assumed that the filter E intercepts the flow of the exhaustgas to increase the pressure and the temperature by dP and dT, Equation(1) is rewritten as:(Pg+dP)·v=R·(Tg+dT)  (3)Equations (2) and (3) give:dT=(Tg·dP)/Pg  (4)According to Equation (4), the temperature increase dT of the filter Eis expected to be proportional to the temperature Tg of the exhaust gasflown into the filter. This is coincident with the measurement resultsshown in FIG. 28(b). Namely the measurement results of FIG. 28(b) provethe validity of the estimated mechanism described above. The phenomenonthat the filter temperature Tf is always higher than the temperature Tgof the exhaust gas at the inlet of the filter is thus ascribed tocompression of the exhaust gas and the resulting increase in temperatureof the exhaust gas when the exhaust gas discharged from the internalcombustion engine passes through the filter.

The increase in temperature Tg of the exhaust gas enhances thetemperature increase dT as clearly understood from Equation (4). Theincrease in flow velocity of the exhaust gas flown into the filterenhances the pressure increase dP and thereby the temperature increasedT of the filter. In general, the temperature of the exhaust gasdischarged from the internal combustion engine is lowered as the flow ofthe exhaust gas passes through the exhaust conduit. The exhaust gas isvigorously ejected from the internal combustion engine to form apulse-like flow having a large flow velocity. As the flow of the exhaustgas passes through the exhaust conduit, the pulse-like flow is averagedto lower the flow velocity. The attachment position of the filter closerto the internal combustion engine raises the temperature and the flowvelocity of the exhaust gas and accordingly enhances the temperatureincrease dT of the filter.

Based on such findings, in the emission control device of the presentinvention and the emission control method corresponding to the emissioncontrol device, a heat insulating section is formed between the exhaustconduit and the emission filter set in the exhaust conduit to interceptthe flow of heat from the filter to the exhaust conduit. Thisarrangement ensures an efficient increase in temperature of the emissionfilter and easy and secure combustion of the carbon-containingparticulates and the hydrocarbon compounds collected on the filter.

In the case where the emission control device is applied to an internalcombustion engine, which includes a plurality of the combustion chambersand an exhaust manifold that unites flows of the exhaust gas from theplurality of combustion chambers to at least one exhaust pipe, onepreferable embodiment sets the emission filter in the exhaust manifold.This locates the emission filter closer to the combustion chamber andenables the high-temperature exhaust gas to be flown into the filter ata high flow velocity, thus effectively increasing the temperature of theemission filter.

In one preferable embodiment of the emission control device, the heatinsulating section is a space formed between the emission filter and theexhaust conduit. The space allows the inflow air to form an air layer orallows the inflow exhaust gas to form an exhaust gas layer, thuseffectively insulating the filter from the exhaust conduit.

In the emission control device of this preferable embodiment, the heatinsulating section may be a space between the emission filter and theexhaust conduit, which has one end open to a flow path of the exhaustgas and is narrowed at the opening. This arrangement enables the exhaustgas to be flown into the space through the opening to the flow path ofhe exhaust gas. The filter is accordingly heated with the hot exhaustgas immediately after the start of the internal combustion engine, sothat the filter temperature is quickly raised. The flow of the exhaustgas is restricted at the narrowed opening and then enters the space.Such restriction desirably prevents the flow of the exhaust gas frombeing vigorously flown into the space to fluidize the exhaust gasexisting inside the space. When the filter temperature becomes higherthan the temperature of the exhaust gas, this arrangement prevents heatfrom being released from the filter to the exhaust conduit due tofluidization of the exhaust gas and thus effectively insulates thefilter from the exhaust conduit.

In the emission control device of this preferable embodiment, the heatinsulating section may be a space between the emission filter and theexhaust conduit, which has one end open to a flow path of the exhaustgas and has a thickness of not greater than 1 mm. In the case where thedistance between the emission filter and the exhaust conduit is 1 mm orless, the existing exhaust gas does not vigorously fluidize inside thespace. Setting the thickness of the space to be not greater than 1 mmthus allows the exhaust gas to be flown into the space through theopening and quickly raise the filter temperature at the time of startingthe internal combustion engine. When the filter temperature isheightened, the setting preferably prevents heat from being releasedfrom the emission filter to the exhaust conduit due to fluidization ofthe exhaust gas existing inside the space.

In still another preferable embodiment of the emission control device,the emission filter is attached to the exhaust conduit via a heatinsulating member. The cooperation of the heat insulating section formedbetween the emission filter and the exhaust conduit with the attachmentof the filter via the heat insulating member more effectively preventsheat from being released from the joint of the filter to the exhaustconduit. This arrangement more effectively insulates the emission filterfrom the exhaust conduit and desirably keeps the filter temperature atthe high level.

In another preferable application of the emission control device, theemission filter has the heat-resistant filter medium for collecting theparticulates in the exhaust gas and a container to receive theheat-resistant filter medium therein. The container is provided with aguide element that leads the exhaust gas discharged from the combustionchamber to the heat-resistant filter medium. The guide element works tolead the flow of the exhaust gas to the heat-resistant filter medium andcauses a large dynamic pressure to be produced in the filter medium,thus effectively raising the filter temperature. The guide element isformed in the container of the filter medium. Even when the flow of theexhaust gas hits against the guide element and releases part of theheat, the structure causes the released heat to be eventually used forheating the emission filter. This arrangement thus ensures an efficientrise of the filter temperature.

In still another preferable application of the emission control device,the emission filter has the heat-resistant filter medium and a containerto receive the heat-resistant filter medium therein. The heat-resistantfilter medium is received in the container such that an end of thefilter medium is projected toward the combustion chamber. Thisarrangement causes the heat produced by the dynamic pressure of theexhaust gas not to be transmitted to the container but to immediatelyraise the temperature at the end of the heat-resistant filter medium.This arrangement thus desirably ensures a quick increase in temperatureof the filter medium.

In the case where the emission control device is applied to an internalcombustion engine provided with a supercharger that utilizesfluidization energy of the exhaust gas to actuate a turbine and therebysupercharge the induction air, one preferable embodiment sets the flowresistance of the emission filter to ½ to ⅔ of the flow resistance ofthe supercharger on a side of the turbine. The flow resistance in thecourse of discharging the exhaust gas mainly depends upon the flowresistance of the supercharger on the side of the turbine. Setting theflow resistance of the emission filter in the range of ½ to ⅔ of theflow resistance of the supercharger on the side of the turbineeffectively prevents a total increase in flow resistance, even when thecarbon-containing particulates are accumulated on the filter to slightlyincrease the flow resistance of the filter.

In the structure where the internal combustion engine is provided with aplurality of combustion chambers and the flows of the exhaust gas fromthe plurality of combustion chambers are united to at least one jointprior to emission, the emission filter may be disposed at the jointwhere the exhaust conduits from the respective combustion chambers aregathered. The arrangement of disposing the emission control device atthe joint where the exhaust conduits from the plurality of combustionchambers are gathered does not require attachment of the emissionfilters to the individual combustion chambers and thus desirably reducesthe total number of the emission filters. There is generally asufficient space for installation of the emission filter at the joint ofthe exhaust conduits. Compared with the structure where the emissionfilters are disposed in the individual combustion chambers, thisarrangement enhances the degree of freedom in shape of the filter anddesirably designs the filter to an optimum shape.

The emission filter may be disposed at a joint where exhaust conduitsfrom all the combustion chambers are united. In one preferableapplication, however, exhaust conduits from every two or threecombustion chambers are gathered to one exhaust port, and the emissionfilter is disposed in each exhaust port. In the structure where all theexhaust conduits are gathered to one joint and the emission filter isdisposed at the joint, there is a relatively large distance between thecombustion chamber and the emission filter. This tends to undesirablylower the inflow temperature of the exhaust gas. In the structure ofthis preferable application where the exhaust conduits from every two orthree combustion chambers are united to one exhaust port, on the otherhand, there is a relatively small distance between the combustionchamber and each exhaust port. Attachment of the emission filter to eachexhaust port effectively prevents a significant decrease in filterinflow temperature of the exhaust gas. This arrangement desirablyenhances the effect of heating the emission filter by means of thedynamic pressure of the exhaust gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the construction of an emission control system, wherea particulate filter of an embodiment is applied to a Diesel engine;

FIG. 2 shows the appearance and the structure of the particulate filterin the embodiment;

FIG. 3 shows a method of manufacturing an element used for theparticulate filter of the embodiment;

FIG. 4 conceptually shows a process of collecting particulates includedin an exhaust gas by means of the particulate filter;

FIG. 5 shows dimensions of non-woven fabrics applicable for theparticulate filter of the embodiment;

FIG. 6 conceptually shows another process of collecting particulatesincluded in the exhaust gas by means of a particulate filter of amodified example;

FIG. 7 shows an attachment structure for fixing the particulate filterto the Diesel engine;

FIG. 8 shows the particulate filter attached to the Diesel engine via afilter holder;

FIG. 9 is a graph showing variations in filter temperature anddifferential pressure before and after the filter when the particulatefilter of the embodiment was applied to the Diesel engine;

FIG. 10 is a graph showing a variation in differential pressure beforeand after the filter when the Diesel engine with the particulate filterof the embodiment attached thereto was driven for a long time period;

FIG. 11 is a graph showing a difference between the pressure variationsin an exhaust conduit before and after the particulate filter;

FIG. 12 is a graph showing a drive pattern of a vehicle in a test;

FIG. 13 is a graph showing measurement results of the filter temperatureand the differential pressure before and after the filter during a runof the 10 lap pattern with the particulate filter of the embodimentapplied to the Diesel engine;

FIG. 14 is a graph showing measurement results of the filter temperatureand the differential pressure before and after the filter during a runof the 11 lap pattern with the particulate filter of the embodimentapplied to the Diesel engine;

FIG. 15 is a graph showing measurement results of the filter temperatureand the differential pressure before and after the filter during a runof an extremely low-speed drive pattern with the particulate filter ofthe embodiment applied to the Diesel engine;

FIG. 16 is a graph showing measurement results of the filter temperatureand the differential pressure before and after the filter during a runof the 10 lap pattern with a particulate filter made of a non-wovenfabric having a pore diameter smaller than an appropriate value;

FIG. 17 shows the composition of an exhaust gas of a Diesel engineincluding carbon-containing particulates and hydrocarbons;

FIG. 18 conceptually shows a process of producing active species througha gentle oxidation reaction of a hydrocarbon with oxygen under atemperature condition lower than the combustible temperature of soot;

FIG. 19 conceptually shows a mechanism of dispersedly collecting sootand other particulates included in the exhaust gas by means of theparticulate filter of the embodiment;

FIG. 20 shows the durability of particulate filters having different manpore diameters;

FIG. 21 conceptually shows a variation in clogging state with avariation in mean pore diameter of the non-woven fabric;

FIG. 22 shows diverse attachment structures of the particulate filter asmodified examples;

FIG. 23 schematically illustrates the structure of a particulate filterin a fourth modified example;

FIG. 24 schematically illustrates the structure of a particulate filterin a fifth modified example;

FIG. 25 shows another emission control system in a sixth modifiedexample;

FIG. 26 shows still another emission control system in a seventhmodified example;

FIG. 27 conceptually shows spontaneous regenerating function of theparticulate filter of the embodiment; and

FIG. 28 shows an estimated principle of converting fluidization energyof the exhaust gas into temperature on the filter.

BEST MODES OF CARRYING OUT THE INVENTION

With a view to further clarifying the functions and the effects of thepresent invention, some modes of carrying out the present invention arediscussed below in the following sequence:

A. System Construction

-   -   A-1. General System Outline    -   A-2. Structure of Particulate Filter    -   A-3. Attachment Structure of Particulate Filter

B. Test Results

-   -   B-1. Results of Engine Bench Test    -   B-2. Results of Vehicle Test    -   B-3. Estimated Mechanism of Spontaneous

Regenerating Function

-   -   B-4. Collection Model    -   B-5. Desired Dimension Ranges of Non-Woven Fabric

C. Modifications

A. System Construction

The following describes an embodiment in which an emission filter of thepresent invention (hereinafter referred to as a particulate filter) isapplied to a Diesel engine. Application is naturally not restricted tothe Diesel engines, but the emission filter of the present invention isapplicable to gasoline engines where a fuel is directly injected into acylinder for combustion and other internal combustion engines. Thetechnique of the present invention is also applicable to any internalcombustion engines for vehicles and ships as well as stationary internalcombustion engines.

A-1. General System Outline

FIG. 1 schematically illustrates the structure of a Diesel engine 10with a particulate filter of the embodiment mounted thereon. The Dieselengine 10 is a 4-cylinder engine and has four combustion chambers #1through #4. The air is supplied to each of the combustion chambers viaan intake pipe 12, while a fuel is injected from a fuel injector 14 setin each combustion chamber. This leads to combustion of the air and thefuel in the combustion chamber, and exhaust gas is discharged through anexhaust manifold 16 to an exhaust pipe 17.

A supercharger 20 is provided in the middle of the exhaust pipe 17. Thesupercharger 20 has a turbine 21 located in the exhaust pipe 17, acompressor 22 set in the intake pipe 12, and a shaft 23 connecting theturbine 21 with the compressor 22. The flow of exhaust gas dischargedfrom the combustion chamber rotates the turbine 21 of the supercharger20 to drive the compressor 22 via the shaft 23. The air is thencompressed and fed into each combustion chamber. An air cleaner 26 isprovided in the upstream of the compressor 22. The compressor 22compresses the intake air through the air cleaner 26 and supplies thecompressed air to the combustion chamber. An inter cooler 24 for coolingdown the air is disposed in the downstream of the compressor 22.Compression of the air by the compressor 22 raises the temperature ofthe air. The compressed air may thus be cooled down by the inter cooler24 and subsequently fed into the combustion chamber. A particulatefilter 100 is provided in each of the combustion chambers #1 through #4in the upstream of the turbine 21. A control unit (hereinafter referredto as control ECU 30) for controlling the engine receives informationrelating to a required output torque, for example, the engine speed andthe accelerator opening, and controls the fuel injector 14, a fuelsupply pump 18, and other diverse actuators (not shown) based on theinput information, thus adequately regulating the driving conditions ofthe Diesel engine.

A-2. Structure of Particulate Filter

FIG. 2 is a perspective view illustrating the appearance of theparticulate filter 100 of the embodiment. With a view to betterunderstanding, part of the cross section is enlarged to show theinternal structure. The particulate filter 100 includes a cylindricalcase 102 with a flange and an element 104 that is inserted in the case102 and has the outer circumference welded to the case 102. The element104 has a rolled cylindrical structure, in which a non-woven fabric 106of a heat-resistant metal and a corrugated sheet 108 of a heat-resistantmetal in piles are rolled up on a core 110. The element 104 used in theparticulate filter 100 of the embodiment has the outer diameter ofapproximately 55 mm and the length of approximately 40 mm. Thesedimensions can appropriately be varied according to the displacement ofthe Diesel engine and the inner diameter of the exhaust conduit.

FIG. 3 conceptually illustrates the process of rolling up the non-wovenfabric 106 and the corrugated sheet 108 in piles on the core 110. Thecorrugated sheet 108 functions to keep the adjoining fragments of therolled-up non-woven fabric 106 at fixed intervals. A large number ofpathways along the axis of the core 110 are accordingly formed betweenthe non-woven fabric 106 and the corrugated sheet 108. Sealing plates112 are welded to both sides of the element 104. The sealing plates 112alternately close the pathways formed between the non-woven fabric 106and the corrugated sheet 108 to define the construction that allows theflow of the exhaust gas to pass through the non-woven fabric 106. Thefunction of the sealing plates 112 to define the construction thatallows passage of the exhaust gas through the non-woven fabric 106 isdiscussed below with reference to FIG. 4.

FIG. 4 conceptually illustrates the sectional structure of theparticulate filter 100. The corrugated sheet 108 is omitted from theillustration of FIG. 4. As clearly shown, the sealing plates 112alternatively close the pathways formed between the adjoining fragmentsof the non-woven fabric 106 kept at fixed intervals. The flow of theexhaust gas from the left side of the drawing as shown by the hatchedarrows in FIG. 4 flows into the pathways that are not closed by thesealing plates 112. The outlets of these pathways are, however, closedby the sealing plates 112. The flow of the exhaust gas accordinglypasses through the non-woven fabric 106 defining the side faces of thepathways and goes to the pathways having the non-closed outlets as shownby the thick arrows. As the flow of the exhaust gas passes through thenon-woven fabric 106, carbon-containing particulates like soot andhydrocarbon compounds included in the exhaust gas are collected by thenon-woven fabric 106.

The non-woven fabric 106 is made of a heat-resistant iron alloy. In theparticulate filter 100 of the embodiment, a metal non-woven fabrichaving dimensions in a predetermined range is applied for the non-wovenfabric 106. This arrangement enables the carbon-containing particulatesand the hydrocarbon compounds to be dispersedly collected in such amanner that brings the respective particulates and hydrocarbon compoundsin contact with oxygen in the exhaust gas. The effects of the dimensionsof the non-woven fabric will be discussed later in detail. Hereexemplified dimensions of the non-woven fabric 106 are shown in FIG. 5.The dimensions of the non-woven fabric shown in FIG. 5 are onlyillustrative and are not restrictive in any sense. A metal non-wovenfabric made of a Fe—Cr—Al alloy is used in this embodiment. Thenon-woven fabric may otherwise be composed of any other knownheat-resistant metals like Ni alloy or ceramic fibers like siliconcarbide fibers.

The reason why the non-woven fabric having the predetermined dimensionsas illustrated in FIG. 5 can dispersedly collect the carbon-containingparticulates and the hydrocarbon compounds in the exhaust gas has notyet been fully elucidated. According to the estimated mechanismdiscussed later, it is expected that not only the metal non-woven fabricbut a ceramic filter having similar dimensions, such as a cordieriteceramic honeycomb filter, gives equivalent results to those of thisembodiment.

In the structure of this embodiment, the sealing plates 112 are weldedto both the sides of the element 104. One possible modification is astructure without the sealing plates 112.

FIG. 6 is a sectional view illustrating the modified structure of theparticulate filter 100 in which the element does not have the sealingplates. For clarity of illustration, the corrugated sheet 108 is omittedfrom the illustration of FIG. 6. In the structure of the embodimentshown in FIG. 4, the sealing plates 112 are alternately welded to boththe sides of the non-woven fabric 106. Instead of welding the sealingplates, the adjoining fragments of the non-woven fabric are welded toeach other at ends 113 in the modified structure shown in FIG. 6. Suchmodified arrangement does not require the sealing plates 112 and thussimplifies the structure of the particulate filter 100.

A-3. Attachment Structure of Particulate Filter

FIG. 7 illustrates an attachment structure for fixing the particulatefilter 100 to the Diesel engine 10. As illustrated, a filter holder 40is provided between a cylinder head 32 defining the upper portion of thecombustion chambers and the exhaust manifold 16. The particulate filters100 are set in the respective combustion chambers in the filter holder40. The filter holder 40 is clamped to the cylinder head 32 with bolts.When the exhaust manifold 16 is clamped with bolts after insertion ofthe particulate filters 100 in the filter holder 40, the flanges areinterposed between the filter holder 40 and the exhaust manifold 16 tofix the particulate filters 100.

FIG. 8 illustrates the particulate filter 100 attached to the Dieselengine 10 via the filter holder 40. As illustrated, the inner diameterof the filter holder 40 is a little greater than the dimensions of theparticulate filter 100. There is accordingly a gap 124 between the outercircumference of the particulate filter 100 and the inner circumferenceof the filter holder 40. The two faces of the flange of the particulatefilter 100 are held between the filter holder 40 and the exhaustmanifold 16 via heat-insulating members 120 and 122 composed of, forexample, glass fibers.

Attachment of the particulate filter 100 to the Diesel engine 10 in thismanner efficiently converts the fluidization energy of the exhaust gasinto heat and raises the filter temperature. In this structure, theparticulate filter 100 is located close to the combustion chamber. Theexhaust gas vigorously ejected from the combustion chambersimultaneously with opening of a exhaust valve of the Diesel engine 10is accordingly flown into the particulate filter 100 without attenuatingthe flow rate and being cooled down. As discussed previously withreference to FIG. 28, the increase of the filter temperature relative tothe temperature of the exhaust gas is enhanced with an increase intemperature of the inflow exhaust gas. The large flow rate enhances thepressure increase of the filter and thereby the increase of the filtertemperature (see Equation (4) given above).

As shown in FIG. 8, there is a gap 124 between the outer circumferentialface of the particulate filter 100 and the inner circumferential face ofthe filter holder 40. The flange of the particulate filter 100 isdisposed via the heat insulating members 120 and 122. This structureeffectively prevents the heat produced in the particulate filter 100from being transmitted to the filter holder 40 and the exhaust manifold16. The arrangement thus enables the heat produced in the particulatefilter 100 to be kept in the filter 100 without being transmitted to thefilter holder 40 and the exhaust manifold 16, thus efficientlyconverting the dynamic pressure of the exhaust gas into the filtertemperature.

It is preferable that the gap 124 defined between the outercircumferential face of the particulate filter 100 and the innercircumferential face of the filter holder 40 is set to be not greaterthan 1 mm. Setting a sufficiently small value, that is, a value of notgreater than 1 mm, to the gap restricts the flow of the exhaust gasinside the gap. As the exhaust gas is fluidized inside the gap 124, theexhaust gas takes heat from the side face of the particulate filter 100and transfers the heat to the filter holder 40. Namely heat, though onlya little quantity, is transmitted from the filter to the filter holder.The gap 124 of not greater than 1 mm effectively prevents transmissionof such a little quantity of heat.

B. Test Results

The following describes the results of various tests where theparticulate filter 100 discussed above is applied to the Diesel engine.

B-1. Results of Engine Bench Test

FIG. 9 is a graph showing variations of the observed differentialpressure before and after the filter, the observed temperature of theexhaust gas flown into the filter, and the observed filter temperatureat the filter outlet, when the particulate filter 100 of the embodimentwas applied to a 4-cylinder Diesel engine and an engine bench test wasperformed under the conditions of steady drive. The abscissa of FIG. 9shows the driving time of the engine. The 4-cylinder Diesel engine of4.3 L displacement was steadily driven under the conditions of theengine speed of 1630 rotations per minute and the torque of 95 Nm. Thetemperature of the exhaust gas was measured at an upstream position ofabout 50 mm apart from the filter with avoiding the effects of radiationfrom the filter.

As shown in the graph of FIG. 9, the temperature of the exhaust gasflown into the particulate filter 100 was substantially fixed at about380° C. The filter temperature, on the other hand, gradually rose andreached 550° C. at approximately 1.5 hours after the start of themeasurement. The filter temperature continued rising and eventuallyreached 575° C. The filter temperature was higher than the temperatureof the exhaust gas flown into the filter by approximately 40° C. to 50°C. immediately after the start of the measurement. This is ascribed tothe conversion of the fluidization energy of the exhaust gas into heatin the particulate filter 100 as described previously.

As mentioned previously, it is thought that the carbon-containingparticulates like soot included in the exhaust gas are subjected tocombustion only at high temperatures of not lower than 550° C. Thegradual increase in filter temperature at the substantially fixedtemperature 380° C. of the exhaust gas flown into the filter may beascribed to the phenomenon that the hydrocarbon compounds other thansoot included in the exhaust gas are collected by the particulate filterand are subjected to an exothermic reaction with oxygen in the exhaustgas. When the filter temperature reaches 550° C., the carbon-containingparticulates like soot collected on the filter start combustion.

The graph of FIG. 9 also shows a variation of the observed differentialpressure before and after the filter. The differential pressure beforeand after the filter increased immediately after the start of themeasurement and was then practically kept unchanged as a whole. Morespecifically, the differential pressure gradually increased immediatelyafter the start of the measurement and tended to decrease when thefilter temperature reached its maximum.

FIG. 10 is a graph showing a variation in differential pressure beforeand after the filter when the operation of the Diesel engine continuedunder the same driving conditions as those of FIG. 9. The differentialpressure before and after the filter was slightly varied but did nottend to increase in the course of the continuous operation of theengine. In general, it is expected that the continuous operation of theDiesel engine with the filter collecting the carbon-containingparticulates in the emission causes the filter to be clogged andincreases the differential pressure unless the collected particulatesare treated by any method. As described above, however, the differentialpressure before and after does not tend to increase in the case of theparticulate filter of the embodiment. It is accordingly thought that thecollected carbon-containing particulates are subjected to combustion onthe filter, irrespective of the low temperature 380° C. of the exhaustgas flown into the filter. When the load of the engine was enhanced at18 hours after the start of the measurement, the differential pressurebefore and after the filter decreased as shown by the arrow in thegraph. The enhanced load raises the temperature of the exhaust gas flowninto the filter and accelerates combustion of the carbon-containingparticulates collected on the filter.

FIG. 11 is a graph showing the observed pressure variations in anexhaust conduit during operations of the Diesel engine. In the Dieselengine, at the moment of opening an exhaust valve, the high-pressureexhaust gas in the combustion chamber is ejected via the exhaust valveand flows in the exhaust conduit as a pressure wave. As shown in FIG.11, there was a significant variation in the upstream of the particulatefilter 100 at the moment of opening the exhaust valve. The pressure wavewas abruptly attenuated, on the other hand, in the downstream of theparticulate filter 100. This shows that the flow of the exhaust gasejected into the exhaust conduit simultaneously with the opening of theexhaust valve is attenuated and converted into heat, while passingthrough the particulate filter 100 of the embodiment.

As described above with reference to FIGS. 9 through 11, the particulatefilter 100 of the embodiment effectively collects the carbon-containingparticulates included in the exhaust gas from the Diesel engine andmakes the collected particulates subjected to combustion without anyspecial control. The estimated mechanism to attain such combustion willbe discussed later.

B-2. Results of Vehicle Test

The following describes the results of tests with the particulate filter100 of the embodiment applied to the Diesel engine mounted on thevehicle. Each of the tests measured the temperature of the exhaust gasflown into the particulate filter 100, the filter temperature, and thedifferential pressure before and after the filter, while driving thevehicle according to a predetermined drive pattern, which repeatedacceleration and deceleration. Two drive patterns, ‘10 lap pattern’ and‘11 lap pattern’ shown in FIG. 12, were mainly used for the tests.

The 10 lap pattern repeats a series of driving pattern, where thevehicle is accelerated from the halt condition to the speed of 60 km perhour and is then decelerated from the speed of 60 km per hour to thehalt condition, 10 times. The 11 lap pattern adds another series ofdriving pattern, where the vehicle is accelerated from the haltcondition to the speed of 100 km per hour and is then decelerated fromthe speed of 100 km per hour to the halt condition, as an 11^(th) lapafter the 10 lap pattern.

(1) Test Results in 10 Lap Run

FIG. 13 is a graph showing measurement results of the temperature of theexhaust gas flown into the particulate filter 100, the filtertemperature, and the differential pressure before and after the filterin the course of a run repeating the 10 lap pattern.

As illustrated, the temperature of the exhaust gas flown into the filtervaried in the range of 370° C. to 400° C. during a run. The filtertemperature was 400° C. immediately after the start of the measurementand gradually increased to about 520° C. The higher filter temperaturethan the temperature of the exhaust gas flown into the filter byapproximately 30° C. immediately after the start of the measurement isascribed to conversion of the fluidization energy of the exhaust gasinto heat in the filter as described previously. The particulate filter100 has the adiabatic structure, so that the filter temperature quicklyrises.

The filter temperature abruptly increased to instantaneously reach 650°C. at the driving distance of approximately 140 km and was quicklylowered to about 470° C. The abrupt increase in filter temperature isascribed to vigorous combustion of the carbon-containing particulatesand the hydrocarbon compounds in the exhaust gas collected on theparticulate filter. The subsequent quick decrease in filter temperatureis attributable to a decrease in quantity of particulates subjected tocombustion. The differential pressure before and after the filter wasabruptly lowered, due to combustion of the collected particulates.

The filter temperature, which was once lowered to 470° C. due tocompletion of the combustion of the carbon-containing particulatescollected on the filter, gradually rose with a further progress of thedrive. The filter temperature reached about 520° C., again abruptly roseto instantaneously reach 600° C. at the driving distance ofapproximately 390 km, and was quickly lowered to about 490° C. Like themoment of the 140 km driving distance, it is expected that thecarbon-containing particulates collected on the particulate filter aresubjected to vigorous combustion at this moment.

The differential pressure before and after the filter, the temperatureof the exhaust gas flown into the filter, and the filter temperaturevaried, immediately after the filter temperature abruptly increased atthe 390 km driving distance and was again lowered to about 490° C. Thisis because the vehicle was driven at the speed of 30 km per hour for 5minutes for the purpose of inspection. Reduction of the vehicle speed tothe level of 30 km per hour decreased the filter temperature to about420° C. As described later, the reactions of the carbon-containingparticulates and the hydrocarbon compounds are kept even under thecondition of the vehicle speed of 30 km per hour.

As shown in FIG. 13, during the run of the 10 lap pattern, collectionand combustion of the carbon-containing particulates and the hydrocarboncompounds in the exhaust gas are repeated on the filter. Thedifferential pressure before and after the filter is slightly variedwith the progress of the repeated collection and combustion, but ispractically stabilized. There is no sign of clogging the filter with theprogress of the run.

(2) Test Results in 11 Lap Run

FIG. 14 is a graph showing measurement results of the filter temperatureof the particulate filter 100 and the differential pressure before andafter the filter in the course of a run repeating the 11 lap pattern.

As mentioned above with reference to FIG. 12, the 11 lap drive patternadds another series of driving pattern, where the vehicle is acceleratedfrom the halt condition to the speed of 100 km per hour and is thendecelerated from the speed of 100 km per hour to the halt condition, asthe 11th lap after the 10 lap pattern. The filter temperaturetemporarily reaches 600° C. during acceleration of the 11^(th) lap. Itis expected that the carbon-containing particulates collected on thefilter are subjected to combustion at this moment. Namely the vehicleruns according to the 11 lap pattern, while the collected particulatesare regularly subjected to combustion during the acceleration of the11^(th) lap. The variations in filter temperature and differentialpressure over the filter with the progress of the drive were measuredbefore and after the acceleration of the 11^(th) lap. The measurementvalues ‘before the acceleration of the 11^(th) lap’ represent theobserved values at the timing of acceleration of the 10^(th) lap, andthe measurement values ‘after the acceleration of the 11^(th) lap’represent the observed values at the timing of acceleration of the1^(st) lap.

The graph of FIG. 14 shows the variations of the filter temperature andthe differential pressure over the filter before and after theacceleration of the 11^(th) lap in the 11 lap run. The closed circlesrepresent the measurement results before the acceleration of the 11^(th)lap (that is, at the time of the acceleration of the 10^(th) lap). Theopen circles represent the measurement results after the acceleration ofthe 11^(th) lap (that is, at the time of the acceleration of the 1^(st)lap). The temperature of the exhaust gas flown into the particulatefilter was substantially fixed to 400° C. For simplicity of explanation,‘before the acceleration of the 11^(th) lap’ and ‘after the accelerationof the 11^(th) lap’ are respectively expressed as ‘before theacceleration to 100 km per hour’ and ‘after the acceleration up to 100km per hour’.

As shown in FIG. 14, the filter temperature and the differentialpressure before and after the filter roughly vary in the followingmanner during the run of the 11 lap pattern. While the differentialpressure after the acceleration to 100 km per hour (expressed by theopen circles) was kept low, the differential pressure after thelow-speed drive of the 10 laps up to the vehicle speed of 60 km per hourand before the acceleration to 100 km per hour (expressed by the closedcircles) gradually increased. The filter temperature reaches 600° C.during acceleration to 100 km per hour, and the collected soot startscombustion. This may be the reason why the filter temperature and thedifferential pressure over the filter decrease after the acceleration to100 km per hour.

The filter temperature prior to acceleration to 100 km per hour(expressed by the closed circles) was approximately 460° C. in the rangeof the driving distance up to 2000 km after the start of the drive,gradually increased in the range of the driving distance from 2000 km to3000 km, and was practically stabilized at temperature exceeding 500° C.in the range of the driving distance after 3000 km. The drive isaccordingly divided into three periods according to the drivingdistance, the initial period up to 2000 km, the transient period between2000 km and 3000 km, and the period after 3000 km. In the initial periodfrom the start of the drive to the driving distance of 2000 km with thelow filter temperature, the hydrocarbon compounds are subjected to thereaction during the low-speed drive of the 10 laps, while thecarbon-containing particulates like soot are subjected to combustionduring the high-speed drive of the 11^(th) lap. In the period of thedriving distance after 3000 km, on the other hand, the filtertemperature intermittently reaches 550° C. even during the low-speeddrive of the 10 laps to start combustion of the collectedcarbon-containing particulates.

The difference between the filter temperature before acceleration to 100km per hour and the filter temperature after the acceleration wasapproximately 20° C. in the initial period from the start of the driveto the driving distance to 2000 km, and increased to about 50° C. in theperiod of the driving distance after 3000 km. This is ascribed topartial combustion of soot even during the low-speed drive of the 10laps in the period of the driving distance after 3000 km.

(3) Test Results in Extremely Low-speed Run

As described previously with reference to FIG. 13, the filtertemperature is abruptly lowered with a decrease in vehicle speed to 30km per hour during the 10 lap run. This, however, does not meanno-combustion of the collected carbon-containing particulates during thelow-speed drive. The collected carbon-containing particulates aresubjected to combustion even during the low-speed drive of 30 km perhour. FIG. 15 is a graph showing measurement results of the filtertemperature during a run of an extremely low-speed drive patternrepeating acceleration and deceleration between the halt condition andthe vehicle speed of 30 km per hour. The temperature of the exhaust gasflown into the filter was approximately 300° C. As illustrated in thegraph, during the extremely low-speed drive, the filter temperaturegradually increased and reached 500° C. at the driving distance of 160km. The filter temperature repeated such a variation, while thedifferential pressure over the filter was stabilized. Namely there is nosign of clogging the filter.

(4) Test Results in Comparative Example

FIG. 16 is a graph showing measurement results of the filter temperatureand the differential pressure before and after the filter during a runwith a particulate filter made of a heat-resistant metal, non-wovenfabric having a distribution of pore diameter smaller than a presetrange, for the purpose of comparison. The run followed the 10 lap drivepattern.

As shown in FIG. 16, when the non-woven fabric of the heat-resistantmetal has the inappropriate distribution of pore diameter, thedifferential pressure before and after the filter exceeds 100 kPa at thedriving distance of 80 km. This breaks the particulate filter. The backpressure increases due to clogging of the filter and thereby raises thetemperature of the exhaust gas flown into the filter. Unlike theparticulate filter of the embodiment, there is no phenomenon that thefilter temperature is gradually apart from the temperature of theexhaust gas flown into the filter. When the non-woven fabric of theheat-resistant metal has the small distribution of pore diameter, thefilter is clogged with the carbon-containing particulates in the exhaustgas. This leads to insufficient supply of oxygen in the exhaust gas tothe collected hydrocarbon compounds, and there are no exothermicreactions of the hydrocarbon compounds with oxygen. When the non-wovenfabric has the appropriate dimensions as in the particulate filter ofthe embodiment, on the other hand, oxygen in the exhaust gas issufficiently supplied to the collected carbon-containing particulatesand hydrocarbon compounds to keep the exothermic reactions of thehydrocarbon compounds with oxygen even in the exhaust gas of therelatively low temperature. With the progress of the reactions, thereaction heat or the active species produced through the reaction areaccumulated to start combustion of the collected carbon-containingparticulates and attain the spontaneous regenerating function.

B-3. Estimated Mechanism of Spontaneous Regenerating Function

The mechanism of the phenomenon occurring in the particulate filter 100of the embodiment, that is, the phenomenon that the carbon-containingparticulates collected on the filter start combustion even in theexhaust gas of the lower temperature than the combustible temperature,has not yet been fully elucidated. The following regards the estimatedmechanism of this spontaneous regenerating function.

As is known, the exhaust gas from the Diesel engine includes thecarbon-containing particulates and the hydrocarbon compounds at a ratioshown in FIG. 17. Roughly speaking, the exhaust gas includes practicallysimilar fractions of the carbon-containing particulates, thefuel-attributed hydrocarbon compounds, and the lubricant oil-attributedhydrocarbon compounds. The carbon-containing particulates like soot arenot subjected to combustion at temperatures of lower than 550° C. evenin the atmosphere of the oxygen-containing exhaust gas. It is expected,on the other hand, that the fuel-attributed hydrocarbon compounds andthe lubricant oil-attributed hydrocarbon compounds are subjected to theoxidation reaction even at temperature of lower than 550° C. under thecondition of a sufficient supply of oxygen.

The phenomenon that the carbon-containing particulates collected on thenon-woven fabric of the heat-resistant metal start combustion even inthe exhaust gas of the lower temperature than the combustibletemperature of the carbon-containing particulates does not occur in thecase of the non-woven fabric having an inappropriate pore diameter or inthe case of a conventionally used ceramic honeycomb filter. As describedpreviously, this phenomenon occurs when the non-woven fabric having thedimensions in the predetermined range is used to collect thecarbon-containing particulates and the hydrocarbon compounds in adispersive manner that brings the respective particulates andhydrocarbon compounds in contact with oxygen included in the exhaustgas. The estimated mechanism of how the non-woven fabric of theembodiment collects the carbon-containing particulates in a dispersivemanner will be discussed later.

There is also a variation in temperature of the non-woven fabric. Whencollection of the carbon-containing particulates and the hydrocarboncompounds continues for a certain time period at temperatures lower thanthe combustible temperature of the carbon-containing particulates, thetemperature of the non-woven fabric gradually increases and eventuallyreaches 550° C., that is, the combustible temperature of thecarbon-containing particulates.

It is accordingly assumed that the following phenomenon occurs on thenon-woven fabric when the particulate filter 100 exerts its spontaneousregenerating function. The carbon-containing particulates and thehydrocarbon compounds in the exhaust gas are collected dispersedly inthe non-woven fabric. The temperature of the exhaust gas flown into thefilter is lower than the combustible temperature of thecarbon-containing particulates. The collected particulates do not thusimmediately start combustion, while the hydrocarbon compounds start somereaction with oxygen in the exhaust gas. Since the filter temperaturegradually rises, this reaction is regarded as a gentle exothermicreaction. When this exothermic reaction continues for a while, thereaction heat is accumulated or the active species produced through thereaction are accumulated to start combustion of the carbon-containingparticulates.

In the case of the cordierite honeycomb filter or the non-woven fabrichaving an inappropriate pore diameter or another dimension, it isdifficult to collect the carbon-containing particulates and thehydrocarbon compounds in a highly dispersive manner to bring therespective particulates and hydrocarbon compounds in contact with oxygenin the exhaust gas. This leads to an insufficient supply of oxygen, andthe gentle exothermic reaction of the hydrocarbon compounds with oxygenin the exhaust gas does not proceed. The reaction heat or the activespecies produced through the reaction are thus not accumulated. Thesefilters accordingly do not exert the spontaneous regenerating function,which is found in the particulate filter of the embodiment.

The higher temperature of the exhaust gas flown into the filter causesthe filter temperature to readily reach the combustible temperature ofthe carbon-containing particulates. Intuitively it is thought that thisleads to easy combustion of the collected carbon-containingparticulates. In the actual state, however, the excessively hightemperature of the exhaust gas flown into the filter may interfere withthe exertion of the spontaneous regenerating function and the resultingcombustion of the carbon-containing particulates. This may be ascribedto that the high temperature of the exhaust gas changes the path of thereaction of the hydrocarbon compounds with oxygen and thereby preventsaccumulation of the active species produced through the reaction.

FIG. 18 conceptually shows simulation of a pre-reaction of a hydrocarboncompound collected on the non-woven fabric with oxygen in the exhaustgas under a temperature condition lower than the combustible temperatureof the carbon-containing particulates. Here the collected hydrocarboncompound is butane (C₄H₁₀), and the chemical reaction of the butanemolecule with oxygen is simulated by computational chemistry. Adiversity of techniques have been proposed for computational chemistry.The technique adopted here is a semi-experimental technique that usesexperimental data for the parts having difficulty in computation andsolves a wave equation representing the electron orbit of the moleculeto trace the chemical reaction.

According to the results of the computation, one hydrogen atom flies outof the butane molecule to start the reaction as shown in FIG. 18(a). Thebutane molecule loosing one hydrogen atom has one unpaired electron, towhich oxygen in the exhaust gas is bonded. The small closed circle inthe drawing represents the place of the unpaired electron. Oxygenextracts one hydrogen atom from a different place when being bonded tothe butane molecule. There is accordingly a new unpaired electron at theposition of the extracted hydrogen atom. Oxygen in the exhaust gas isalso bonded to the position of the new unpaired electron. The butanemolecule is gradually bonded to oxygen in the exhaust gas in this mannerto produce a partly oxidized active species. This oxidation reaction isexothermic. As the reaction proceeds, the temperature rises and thepartly oxidized active species is accumulated.

As the reaction in the first stage proceeds, the temperature rises andthe partly oxidized active species is accumulated. The reaction thengoes to a second stage, where an OH radical is produced from the activespecies obtained by partial oxidation of the butane molecule as shown inFIG. 18(b). The OH radical is highly reactive and causes abruptcombustion of the remaining hydrocarbon compounds and carbon-containingparticulates.

According to the results of the computation, sufficient accumulation ofthe partially oxidized active species in the first stage of the reactionleads to production of a greater quantity of the OH radical in thesecond stage of the reaction, thus causing abrupt combustion of theremaining hydrocarbon compounds and carbon-containing particulates. Ahydrocarbon compound containing a greater number of carbon atoms thanthe butane molecule enables a greater number of oxygen atoms to bebonded to one molecule. This leads to production of a greater quantityof the OH radical for the abrupt combustion. In the second term shown inFIG. 27 described previously, the hydrocarbon compounds and oxygenincluded in the exhaust gas undergo reactions as shown in FIG. 18 on theparticulate filter. When the reaction heat or the active speciesproduced through the reaction are sufficiently accumulated, thecarbon-containing particulates collected on the filter start combustion.

B-4. Collection Model

As described above, the particulate filter 100 of the embodiment iscomposed of the non-woven fabric having the predetermined dimensions andis thus capable of dispersedly collecting the carbon-containingparticulates and the hydrocarbon compounds in the exhaust gas. Themechanism discussed below is estimated to actively take and collect theparticulates like soot in the non-woven fabric. The estimated collectionmechanism is briefly described below.

FIG. 19 conceptually shows the cross sectional structure of a non-wovenfabric of a heat-resistant metal. The hatched circles in the drawingrespectively represent the cross sections of fibers of the non-wovenfabric. The non-woven fabric is composed of numerous fibers tangledintricately and has numerous three-dimensional pathways connecting withone another in a complicated manner.

FIG. 19(a) conceptually shows the cross sectional structure of a newnon-woven fabric. It is here assumed that the exhaust gas flows down.Because of the variation in distribution of fibers, opening of varioussizes are formed on the surface of the non-woven fabric. Even the smallopening is sufficiently large for the gas molecules in the exhaust gas.The flow of the exhaust gas thus passes through the whole surface of thenon-woven fabric in a practically uniform manner. In the drawing of FIG.19(a), the flows of the exhaust gas between the fibers of the non-wovenfabric are schematically expressed by the thick arrows.

As the flow of the exhaust gas passes through the non-woven fabric, theparticulates like soot included in the exhaust gas are trapped betweenthe fibers and gradually clog the openings on the surface of thenon-woven fabric. The small openings on the surface of the non-wovenfabric are clogged with the particulates like soot, and the flows of theexhaust gas go to the non-clogged but remaining, relatively largeopenings as shown in FIG. 19(b). The flows of the exhaust gas passingthrough the non-woven fabric accordingly meet together to the flows fromthe non-clogged but remaining, relatively large openings on the surface.In the drawing of FIG. 19(b), the particulates like soot areschematically expressed by the small closed circles.

The integrated flow of the exhaust gas increases the flow velocity andcauses a significant pressure gradient in the pathway. This phenomenonmay be compared to the collision of the flow against the fibers of thenon-woven fabric to produce a large pressure. As mentioned previously,the pathways formed inside the non-woven fabric communicate with oneanother in a complicated manner. The higher pressure of the integratedflow in the pathway causes the flow to immediately branch off to theother pathways. The differential pressure before and after the non-wovenfabric thus does not increase to or above a preset level but is kept ina fixed range.

FIG. 19(c) conceptually shows the main stream branching off to the otherpathways. As the flow of the exhaust gas branches off in the non-wovenfabric, the carbon-containing particulates like soot included in theexhaust gas are collected by the whole area of the non-woven fabric.Even if a certain place in the non-woven fabric is clogged with soot,the three-dimensional connection of the pathways allows the flow toimmediately branch off to the other pathways. Namely even when a certainplace in the non-woven fabric is clogged with soot and the otherparticulates, the flow path of the exhaust gas is automatically changedto new pathways. This arrangement ensures dispersed collection of thesoot and the other particulates.

As described above, the particulate filter 100 of the embodiment is usedfor collection of the carbon-containing particulates like soot includedin the exhaust gas and spontaneous combustion of the collected soot andother particulates. This arrangement easily regulates and reduces thecarbon-containing particulates included in the exhaust gas without anyspecial control.

Since the collected carbon-containing particulates spontaneously startcombustion, no labor-consuming treatment is required to estimate thecollection state of the soot and start combustion. This arrangementensures the effective regulation and reduction of the carbon-containingparticulates in the exhaust gas.

The collected carbon-containing particulates like soot and hydrocarboncompounds spontaneously start combustion, so that there is nopossibility of clogging and breaking the filter.

Simple attachment of the particulate filter of the embodiment into theexhaust pipe of the conventional internal combustion engine gives theextremely simple but highly reliable emission control system. The simpleattachment of the particulate filter also significantly reduces themanufacturing cost of the emission control system.

B-5. Desired Dimension Ranges of Non-woven Fabric

As described previously, the particulate filter 100 of the embodimentcollects the carbon-containing particulates and the hydrocarboncompounds in the exhaust gas in a dispersive manner that brings therespective particulates and hydrocarbon compounds in contact with oxygenin the exhaust gas. This allows combustion of the collected particulatesin the exhaust gas of the lower temperature than the combustibletemperature of the carbon-containing particulates. This phenomenon isnot observed in the case of the metal non-woven fabric having the smallpore diameter or the cordierite honeycomb filter as described above. Forsuccessful exertion of the spontaneous regenerating function, thedimensions of the non-woven fabric should be in a preset range. Furthersystematic experiments are required to specify the preset range.According to the results of the experiments performed so far, it is atleast preferable that the mean inner diameter of the pores formed insidethe non-woven fabric is in a range of approximately 5 μm toapproximately 25 μm. The reason of such specification is discussedbelow.

As described above with reference to FIG. 19, in the particulate filter100 of the embodiment, the flow path of the exhaust gas is automaticallychanged in the non-woven fabric to dispersedly collect thecarbon-containing particulates and the hydrocarbon compounds. In orderto allow the change of the flow path of the exhaust gas, the non-cloggedlarge openings should be present at a certain ratio to the openingsformed by the pores on the surface of the non-woven fabric, while thesmall openings are clogged with the collected carbon-containingparticulates (see FIGS. 19(b) and 19(c)). Since the large openingsshould be present at a certain ratio, the mean pore diameter of thenon-woven fabric is required to be greater than a preset value. In thetest with a non-woven fabric having the mean pore diameter of 5 μm, theparticulate filter was soon clogged. It is accordingly preferable thatthe mean pore diameter of the non-woven fabric is greater than about 5μm. The test results shown in FIG. 16 support such specification.

When the non-woven fabric has an extremely large mean pore diameter, onthe contrary, the openings on the surface of the non-woven fabric arehardly clogged. The flow path of the exhaust gas is thus not changed inthe non-woven fabric. This proves that the mean pore diameter of thenon-woven fabric should be smaller than a preset value. In the test witha non-woven fabric having the mean pore diameter of 25 μm, unlike theparticulate filter of the embodiment, no spontaneous regeneratingfunction was observed. It is accordingly preferable that the mean porediameter is smaller than about 25 μm.

In the specification hereof, the mean pore diameter represents the meanvalue of the pore diameter measured according to the Washburn'sequation. When the filter is soaked in a liquid, the smaller porediameter enhances the possibility of clogging the pores by the surfacetension of the liquid and thereby increases the air-flow resistance ofthe filter. The Washburn's equation notes this phenomenon and specifiesthe relation of the differential pressure before and after the filterwith the surface tension of a liquid, the contact angle of the liquidwith the filter, and the pore diameter of the filter. The measurementaccording to the Washburn's equation is widely used to obtain thedistribution of pore diameter and is not specifically described here.Namely the mean pore diameter is the pore diameter having theaccumulated pore volume of 50% measured according to the Washburn'sequation. The numerical value of the mean pore diameter is varied inmeasurement of another known method.

From the viewpoint of the durability of the particulate filter 100, themean pore diameter of the non-woven fabric is preferably in a range ofabout 11 μm to about 13 μm. The following describes the reason of suchspecification.

Deterioration of the particulate filter 100 may be caused byaccumulation of the particulates, called ash, on the filter. Metalcomponents like Ca, G, and Zn included in additives of engine oil arecombined with sulfur in the fuel to form sulfates. The sulfates depositas the ash. The metal sulfates are thermally stable. The ash accumulatedon the filter is not subjected to combustion unlike thecarbon-containing particulates, but clogs the particulate filter 100.With a view to evaluating the durability to the ash, the durability testof various particulate filters having different mean pore diameters wasperformed with a Diesel engine having the intentionally increasedquantity of ash. More specifically, the spontaneous regeneratingfunction was evaluated after 20-hour duration under the total loadingcondition with an engine having the 5-fold consumption of engine oil.

FIG. 20 shows the summary of the test results. The test evaluated threeparticulate filters of non-woven fabrics having the mean pore diameterof 10 μm, 12 μm, and 14 μm. Any of the new particulate filters has thespontaneous regenerating function. The ‘double circle’ in FIG. 20 showsthat the favorable spontaneous regenerating function has been observed.The particulate filter having the mean pore diameter of 12 μm had thefavorable spontaneous regenerating function even after the durationtest. Both the filter having the mean pore diameter of 10 μm and thefilter having the mean pore diameter of 14 μm were, on the other hand,clogged in the course of the duration test and broken during theevaluation. The spontaneous regenerating function was accordingly notobservable in either case.

After the duration test, the non-woven fabric of each filter wasobserved with an optical microscope. In the non-woven fabric having themean pore diameter of 10 μm, the fibers were still observable on theoutflow surface of the exhaust gas (the surface at the outlet), whereasthe fibers were hardly observable on the inflow surface of the exhaustgas (the surface at the inlet) that was clogged with ash and hadaccumulation of the carbon-containing particulates. In the non-wovenfabric having the mean pore diameter of 14 μm, on the contrary, thesurface at the inlet was not clogged, whereas the openings formedbetween the fibers on the surface at the outlet were clogged by the ashand the carbon-containing particulates. In the non-woven fabric havingthe mean pore diameter of 12 μm, the ash adhered to some fibers of thenon-woven fabric on both the surface at the inlet and the surface at theoutlet. The openings between the fibers were, however, not at allclogged by the ash.

FIG. 21 conceptually shows a variation in clogging state with avariation in mean pore diameter of the non-woven fabric. The drawingshows the cross section of the particulate filter 100 along part of thepathways formed between the non-woven fabric 106 and the corrugatedsheet 108 of the particulate filter 100. The exhaust gas is flown fromthe left side of the drawing into the filter, passes through thenon-woven fabric 106 as shown by the arrows, and goes to the right sideof the drawing. The filled parts schematically represent accumulation ofthe ash and the carbon-containing particulates on the surface of thenon-woven fabric.

FIG. 21(a) shows the particulate filter having the mean pore diameter of10 μm. In the case of the non-woven fabric having the mean pore diameterof 10 μm, the ash and the carbon-containing particulates are locallyaccumulated on the surface of the non-woven fabric at the inlet, thatis, on the inflow surface of the exhaust gas to clog the pores of thenon-woven fabric. FIG. 21(b) shows the particulate filter having themean pore diameter of 12 μm. In the case of the non-woven fabric havingthe mean pore diameter of 12 μm, the ash is evenly dispersed over thewhole surface of the non-woven fabric, while the spontaneousregenerating function causes combustion of the carbon-containingparticulates. The carbon-containing particulates are thus notaccumulated to any significant depth and do not clog the pores. FIG.21(c) shows the particulate filter having the mean pore diameter of 14μm. In the case of the non-woven fabric having the mean pore diameter of14 μm, the ash and the carbon-containing particulates are notaccumulated to any significant depot on the surface at the inlet, butare locally accumulated on the surface at the outlet, that is, theoutflow surface of the exhaust gas, to clog the pores of the non-wovenfabric.

The greater mean pore diameter of the non-woven fabric leads tolocalization and accumulation of the ash and the carbon-containingparticulates on the surface of the non-woven fabric at the outlet. Thismay be ascribed to the fact that the greater pore diameter makes it moredifficult to change over the flow path of the exhaust gas passingthrough the non-woven fabric. As discussed previously with reference toFIG. 19, the particulate filter 100 of the embodiment dispersedlycollects the carbon-containing particulates by changing over the flowpath of the exhaust gas in the non-woven fabric. When the non-wovenfabric has a large mean pore diameter, the carbon-containingparticulates and the ash are not collected on the surface of thenon-woven fabric at the inlet. No changeover of the flow path of theexhaust gas, however, results in localization and accumulation of theash and the carbon-containing particulates in the vicinity of theoutlet.

As described above, when the mean pore diameter of the non-woven fabricis not greater than 10 μm, the ash and the carbon-containingparticulates may accumulate on the surface of the non-woven fabric atthe inlet to clog the particulate filter after the long-time use. Whenthe mean pore diameter of the non-woven fabric is not less than 14 μm,on the other hand, the ash and the carbon-containing particulates mayaccumulate on the surface of the non-woven fabric at the outlet to clogthe particulate filter after the long-time use. It is accordinglypreferable that the non-woven fabric of the particulate filter has themean pore diameter in the range of about 11 μm to 13 μm. Someexperiments showed the best results when the non-woven fabric had themean pore diameter of 12 μm±10%.

The above description regards the mean pore diameter of the non-wovenfabric. Setting the pore diameter of the non-woven fabric in the aboverange automatically specifies the desirable range of the fiber diameterof the non-woven fabric. The greater pore diameter lowers the fiberdensity in the non-woven fabric. The less fiber density lowers thestrength of the non-woven fabric. In order to compensate for the loweredstrength due to the less fiber density, each fiber should have thegreater diameter. To ensure the sufficient strength of the non-wovenfabric, the greater pore diameter leads to the greater fiber diameter.It is also empirically known that the factors on manufacture of thenon-woven fabric often cause the fiber diameter to increase with anincrease in pore diameter. Because of these reasons, it is desirablethat the fiber diameter is not less than about 15 μm and not greaterthan about 20 μm to attain the mean pore diameter of the non-wovenfabric in the range of about 11 μm to 13 μm.

C. Modifications

The emission control device discussed above may be modified in variousways. FIG. 22 shows modified examples of the attachment structure of theparticulate filter 100. The following describes these modified examples.

(1) First Modified Example

FIG. 22(a) shows an attachment structure of a first modified example. Inthe first modified example, a projection 126 is formed on the innercircumferential face of the filter holder 40 at a position correspondingto the inlet to the gap 124. The projection 126 formed on the innercircumferential face of the filter holder 40 intercepts the flow of theexhaust gas and prevents the direct flow of the exhaust gas into the gap124. The arrangement thus restricts the flow of the exhaust gas insidethe gap 124. This decreases the quantity of heat transmission to thefilter holder 40 and thereby keeps the particulate filter 100 atsufficiently high temperature.

(2) Second Modified Example

FIG. 22(b) shows an attachment structure of a second modified example.In the second modified example, a step is formed on the innercircumferential face of the filter holder 40 to interfere with thedirect flow of the exhaust gas into the gap 124. This arrangement thusrestricts the flow of the exhaust gas inside the gap 124.

In the second modified example, the step formed on the innercircumferential face of the filter holder 40 is close to the end of theparticulate filter 100. A restriction 128 is accordingly providedbetween the end of the filter and the step. The restriction 128functions to interfere with the inflow of the exhaust gas and restrictsthe flow of the exhaust gas inside the gap 124. This decreases thequantity of heat transmission to the filter holder 40 and thereby keepsthe particulate filter 100 at sufficiently high temperature.

(3) Third Modified Example

FIG. 22(c) shows an attachment structure of a third modified example. Inthe third modified example, a heat insulating member 130 is set on theouter circumference at the end of the particulate filter 100. The gap124 is defined behind the heat insulating member 130 in attachment ofthe particulate filter 100. In the attachment structure of the thirdmodified example, the heat insulating member 130 intercepts the flow ofthe exhaust gas into the gap 124 and thus keeps the particulate filter100 at high temperature.

In the structure of the third modified example, the heat insulatingmember 130 guides the particulate filter 100 and keeps the particulatefilter 100 apart from the filter holder 40 at a fixed interval. Thisadvantageously facilitates attachment of the filter.

In the structure of the third modified example, the heat insulatingmember 130 is disposed only on the outer circumference at the end of theparticulate filter 100. The heat insulating member 130 may otherwise beset over the whole outer circumference.

(4) Fourth Modified Example

The particulate filter 100 of a modified structure may be applied forthe emission control device of the embodiment. FIG. 23 schematicallyshows the structure of the particulate filter 100 used in a fourthmodified example. For better understanding, the cross section of theparticulate filter 100 is partly illustrated in FIG. 23. The particulatefilter 100 of the fourth modified example has a guide element 103 on thecase 102. The guide element 103 leads the flow of the exhaust gas intothe particulate filter 100. This arrangement enables the filtertemperature to be efficiently raised by effectively utilizing thedynamic pressure of the exhaust gas.

In the structure of the fourth modified example, the guide element 103is provided on the case 102 of the particulate filter 100 and is apartfrom the filter holder 40 via the gap 124. Part of the thermal energy ofthe exhaust gas may be transferred to the guide element 103, when theexhaust gas is led by the guide element 103 and is flown into theparticulate filter 100. This arrangement prevents the transferredthermal energy from being released to the filter holder 40 but causesthe thermal energy to be used to raise the filter temperature, thuskeeping the particulate filter 100 at high temperature.

(5) Fifth Modified Example

FIG. 24 schematically shows the structure of the particulate filter 100used in a fifth modified example. For better understanding, the crosssection of the particulate filter 100 is partly illustrated in FIG. 24.In the particulate filter 100 of the fifth modified example, the end ofthe element 104 is extended from the case 102. This arrangement preventsrelease of heat from the filter to the case 102 at the end in theprocess of converting the dynamic pressure of the exhaust gas into heatand thus ensures a quick temperature rise at the end of the filter. Thetemperature gradient inside the filter enables the end of the filter tobe kept at higher temperature than the temperature of the other part ofthe filter.

(6) Sixth Modified Example

In the structures of the embodiment and modified examples discussedabove, the particulate filter 100 is provided for each combustionchamber. It is, however, not essential to provide the particulate filter100 for each combustion chamber. In one possible modification, the flowof the exhaust gas from multiple combustion chambers is gathered to ajoint flow, and the particulate filter is disposed at the joint. In anexample shown in FIG. 25, the flow of the exhaust gas from every two orthree cylinders is gathered to a joint flow, and the particulate filteris disposed at each joint. There may be not a sufficient space forsetting the filter in the vicinity of the combustion chamber. It isrelatively easy to find the sufficient space at the joint. Thisarrangement thus allows the particulate filter 100 to have a greatersize or an optimum shape.

(7) Seventh Modified Example

In any of the embodiment and modified examples discussed above, theparticulate filter 100 may be combined with a NOx reduction catalyst oranother catalyst. This application is discussed below as a seventhmodified example.

FIG. 26 illustrates an emission control system of the seventh modifiedexample where the particulate filter 100 of the embodiment is combinedwith a NOx absorbing reduction-type three-way catalyst 200. The NOxabsorbing reduction-type three-way catalyst 200 absorbs nitrogen oxidesin the exhaust gas under the condition that excess oxygen is present inthe exhaust gas, and starts reduction of the absorbed nitrogen oxideswith the hydrocarbon compounds and carbon monoxide included in theexhaust gas with a decrease in concentration of oxygen in the exhaustgas. The iterative absorption and reduction of the nitrogen oxideseffectively regulates and diminishes the nitrogen oxides in the exhaustgas.

In the emission control system shown in FIG. 26, the carbon-containingparticulates included in the exhaust gas are regulated and diminished inthe upstream of the NOx absorbing reduction-type three-way catalyst 200.This arrangement desirably avoids the potential problem of thedownstream NOx absorbing reduction-type three-way catalyst 200 that iscovered with soot to lower the performances.

The NOx absorbing reduction-type three-way catalyst 200 releases activeoxygen in the process of absorbing nitrogen oxides in the exhaust gas orin the process of reducing the absorbed nitrogen oxides. The activeoxygen has extremely high reactivity and readily causes combustion ofthe carbon-containing particulates. The combined use of the NOxabsorbing reduction-type three-way catalyst 200 in the downstream of theparticulate filter 100 thus ensures regulation and omission of thecarbon-containing particulates in the downstream NOx absorbingreduction-type three-way catalyst 200 even when the carbon-containingparticulates pass through the particulate filter 100.

In some cases, the particulate filter of the embodiment may be disposedin the downstream of the NOx absorbing reduction-type three-way catalyst200 to effectively regulate and diminish nitrogen oxides and soot in theexhaust gas. For example, in the case of relatively little emission ofthe soot but significantly large emission of the nitrogen oxides, theNOx absorbing reduction-type three-way catalyst 200 is disposed in theupstream of the particulate filter. This arrangement feeds thehydrocarbon compounds immediately after the start of reduction of thenitrogen oxides, thus quickly diminishing the nitrogen oxides.

The above embodiment and its modifications are to be considered in allaspects as illustrative and not restrictive. There may be manymodifications, changes, and alterations without departing from the scopeor spirit of the main characteristics of the present invention. Allchanges within the meaning and range of equivalency of the claims aretherefore intended to be embraced therein.

For example, in the above embodiment and modified examples, the air maybe supplied to the upstream of the particulate filter 100 by means of alead valve or a power-driven pump. The supply of oxygen to the exhaustgas according to the requirements favorably accelerates the reaction ofthe collected hydrocarbon compounds with oxygen.

A metal catalyst having appropriate oxidation activity, such as Fe, Cu,and Co, may be carried on the non-woven fabric. Such a metal catalystaccelerates the reaction of the collected hydrocarbon compounds withoxygen in the low-temperature exhaust gas and favorably ensurescombustion of the collected carbon-containing particulates.

In the structures of the above embodiment and modified examples, theparticulate filter is attached by means of the filter holder 40. Theparticulate filter may, however, be directly inserted into and fixed tothe exhaust port of the cylinder head without using the filter holder40. The particulate filter may otherwise be inserted into and fixed tothe downstream exhaust manifold 16.

Industrial Applicability

As described above, the emission control device of the present inventionensures easy and secure regulation and reduction of thecarbon-containing particulates in the exhaust gas over a long timeperiod without requiring any specific control to raise the temperatureof the exhaust gas or using any precious noble metals. The technique ofthe present invention is thus preferably applied to the emission filtersfor controlling the emission from various internal combustion engines,the emission control devices with such a filter, as well as internalcombustion engines with the emission control device for diverse vehiclesand ships and stationary internal combustion engines.

1. An emission filter for regulating and reducing carbon-containingparticulates included in an exhaust gas from an internal combustionengine, said emission filter comprising: a heat-resistant filter mediumthat collects a hydrocarbon compound and the carbon-containingparticulates included in the exhaust gas in a dispersive manner to bringthe respective particulates and hydrocarbon compound in contact withoxygen included in the exhaust gas, and thereby makes the collectedhydrocarbon compound and the collected carbon-containing particulatessubjected to combustion with the exhaust gas having a filter inflowtemperature lower than a combustible temperature of thecarbon-containing particulates without requiring use of catalyzingagents, wherein said heat-resistant filter medium utilizes heat ofreaction of the collected hydrocarbon compound with oxygen included inthe exhaust gas, so as to make the collected carbon-containingparticulates subjected to combustion, said heat-resistant filter mediumincludes multiple pathways, which connect with one another in athree-dimensional manner inside said heat-resistant filter medium andare open to surface of said heat-resistant filter medium, and themultiple pathways included in said heat-resistant filter medium have amean inner diameter in a range of 11 μm to 13 μm, the subject filterbeing self-reproducing without heating by a heater.
 2. An emissionfilter in accordance with claim 1, wherein said heat-resistant filtermedium utilizes active species produced by the reaction of the collectedhydrocarbon compound with oxygen included in the exhaust gas, inaddition to the heat of the reaction, so as to make the collectedcarbon-containing particulates subjected to combustion.
 3. An emissionfilter in accordance with claim 1, wherein said heat-resistant filtermedium traps the carbon-containing particulates and the hydrocarboncompound therein.
 4. An emission filter in accordance with claim 3,wherein said heat-resistant filter medium utilizes a variation inpressure of the exhaust gas from said internal combustion engine todispersedly collect the carbon-containing particulates and thehydrocarbon compound.
 5. An emission filter in accordance with claim 1,wherein said heat-resistant filter medium converts fluidization energyof the exhaust gas from said internal combustion engine into heat, so asto raise the own temperature of said heat-resistant filter medium.
 6. Anemission filter in accordance with claim 5, wherein said heat-resistantfilter medium is heated by utilizing a temperature rise in the processof compressing the exhaust gas by means of a dynamic pressure.
 7. Anemission filter in accordance with claim 1, wherein said heat-resistantfilter medium is a non-woven fabric made of heat-resistant fibers havinga mean fiber diameter in a range of 15 μm to 20 μm.
 8. An emissionfilter in accordance with claim 7, wherein said heat-resistant filtermedium is a non-woven fabric having a thickness in a range of 0.4 mm to0.5 mm.
 9. An emission filter in accordance with claim 1, wherein saidheat-resistant filter medium changes over a flow path of the exhaust gasflowing through the multiple pathways in the course of collecting thecarbon-containing particulates and the hydrocarbon compound.
 10. Anemission filter in accordance with claim 9, wherein said heat-resistantfilter medium changes over the flow path of the exhaust gas flowingthrough the multiple pathways when a pressure loss in the course of thecollection reaches three to four times of an initial value.
 11. Anemission control device that is applied to an internal combustion engineto regulate and reduce carbon-containing particulates included in anexhaust gas, wherein said internal combustion engine comprises acombustion chamber and an exhaust conduit for discharging the exhaustgas from said combustion chamber, said emission control devicecomprising: an emission filter that is attached to said exhaust conduitto collect the carbon-containing particulates included in the exhaustgas; and a heat insulating section that is interposed between saidemission filter and said exhaust conduit, such that said heat insulatingsection is a space formed between said emission filter and said exhaustconduit, wherein said emission filter comprises a heat-resistant filtermedium that collects a hydrocarbon compound and the carbon-containingparticulates included in the exhaust gas in a dispersive manner to bringthe respective particulates and hydrocarbon compound in contact withoxygen included in the exhaust gas, and thereby makes the collectedhydrocarbon compound and the collected carbon-containing particulatessubjected to combustion, without requiring use of catalyzing agents,with the exhaust gas having a filter inflow temperature lower than acombustible temperature of the carbon-containing particulates, thesubject filter being self-reproducing without heating by a heater. 12.An emission control device in accordance with claim 11, wherein saidinternal combustion engine is provided with a plurality of saidcombustion chambers and an exhaust manifold that unites flows of theexhaust gas from said plurality of combustion chambers to at least oneexhaust pipe, and said emission filter is disposed in said exhaustmanifold.
 13. An emission control device in accordance with claim 11,wherein said heat insulating section has one end open to a flow path ofthe exhaust gas and is narrowed at the opening.
 14. An emission controldevice in accordance with claim 11, wherein said heat insulating sectionhas one end open to a flow path of the exhaust gas and has a thicknessof not greater than 1 mm.
 15. An emission control device in accordancewith claim 11, wherein said emission filter is attached to said exhaustconduit via a heat insulating member.
 16. An emission control device inaccordance with claim 11, wherein said emission filter has a containerto receive said heat-resistant filter medium therein, and said containeris provided with a guide element that leads the exhaust gas dischargedfrom said combustion chamber to said heat-resistant filter medium. 17.An emission control device in accordance with claim 11, wherein saidemission filter has a container to receive said heat-resistant filtermedium therein, and said heat-resistant filter medium is received insaid container such that an end of said filter medium is projectedtoward said combustion chamber.
 18. An emission control device inaccordance with claim 11, wherein said internal combustion enginecomprises a supercharger that utilizes fluidization energy of theexhaust gas to actuate a turbine, so as to supercharge induction air ofsaid internal combustion engine, and said emission filter has a flowresistance that is ½ to ⅔ of a flow resistance of said supercharger on aside of said turbine.
 19. An emission control device that is applied toan internal combustion engine to regulate and reduce carbon-containingparticulates included in an exhaust gas, where said internal combustionengine comprises a plurality of combustion chambers and unites flows ofthe exhaust gas from said plurality of combustion chambers to at leastone joint, prior to emission, said emission control device comprising:an emission filter that is disposed at said at least one joint, wherethe flows of the exhaust gas from said plurality of combustion chambersare united, to collect a hydrocarbon compound and the carbon-containingparticulates included in the exhaust gas, wherein said emission filtercomprises a heat-resistant filter medium that collects a hydrocarboncompound and the carbon-containing particulates included in the exhaustgas in a dispersive manner to bring the respective particulates andhydrocarbon compound in contact with oxygen included in the exhaust gas,and thereby makes the collected hydrocarbon compound and the collectedcarbon-containing particulates subjected to combustion with the exhaustgas having a filter inflow temperature lower than a combustibletemperature of the carbon-containing particulates, wherein saidheat-resistant filter medium utilizes heat of reaction of the collectedhydrocarbon compound with oxygen included in the exhaust gas withoutrequiring use of catalyzing agents, so as to make the collectedcarbon-containing particulates subjected to combustion, the subjectfilter being self-reproducing without heating by a heater.
 20. Anemission control device in accordance with claim 19, wherein saidinternal combustion engine unites flows of the exhaust gas dischargedfrom every two or three combustion chambers to one exhaust port, priorto emission, and said emission filter is disposed at every exhaust port,where the flows of the exhaust gas discharged from every two or threecombustion chambers are united.
 21. An emission control method thatregulates and reduces carbon-containing particulates included in anexhaust gas of an internal combustion engine, said emission controlmethod comprising the steps of: utilizing a heat-resistant filter mediumto collect a hydrocarbon compound and the carbon-containing particulatesincluded in the exhaust gas in a dispersive manner to bring therespective particulates and hydrocarbon compound in contact with oxygenincluded in the exhaust gas; and making the collected hydrocarboncompound and the collected carbon-containing particulates subjected tocombustion with the exhaust gas having a filter inflow temperature lowerthan a combustible temperature of the carbon-containing particulates, soas to regulate and reduce the carbon-containing particulates, saidheat-resistant filter medium utilizing heat of reaction of the collectedhydrocarbon compound with oxygen included in the exhaust gas withoutrequiring use of catalyzing agents, so as to make the collectedcarbon-containing particulates subjected to combustion, the subjectfilter being self-reproducing without heating by a heater.
 22. Anemission control method in accordance with claim 21, said emissioncontrol method further comprising the step of: leading a supply ofoxygen into the exhaust gas, so as to accelerate a reaction of at leasteither of the collected hydrocarbon compound and the collectedcarbon-containing particulates with oxygen.
 23. An emission controlmethod in accordance with claim 21, said emission control method furthercomprising the step of: arranging a NOx reduction catalyst in downstreamof said heat-resistant filter medium to diminish nitrogen oxidesincluded in the exhaust gas.
 24. An emission control method inaccordance with claim 23, wherein the NOx reduction catalyst is acatalyst that absorbs the nitrogen oxides under a condition that excessoxygen is present in the exhaust gas, and reduces the absorbed nitrogenoxides with a decrease in concentration of oxygen in the exhaust gas, soas to diminish the nitrogen oxides in the exhaust gas.
 25. An emissioncontrol method that is applied to an internal combustion engine toregulate and reduce carbon-containing particulates included in anexhaust gas, where said internal combustion engine comprises acombustion chamber and an exhaust conduit for discharging the exhaustgas from said combustion chamber, said emission control methodcomprising the steps of: disposing an emission filter, which comprises aheat-resistant filter medium, in said exhaust conduit in such a mannerthat a heat insulating section is formed between said emission filterand said exhaust conduit; utilizing said heat-resistant filter medium tocollect a hydrocarbon compound and the carbon-containing particulatesincluded in the exhaust gas in a dispersive manner to bring therespective particulates and hydrocarbon compound in contact with oxygenincluded in the exhaust gas; and making the collected hydrocarboncompound and the collected carbon-containing particulates subjected tocombustion with the exhaust gas having a filter inflow temperature lowerthan a combustible temperature of the carbon-containing particulateswithout requiring use of catalyzing agents, so as to regulate and reducethe carbon-containing particulates, wherein the emission filter isattached to the exhaust conduit via a heat insulating section, so as toprevent the exhaust gas flowing in the exhaust conduit from taking awaythe heat generated in the filter, the filter being self-reproducingwithout heating by a heater.
 26. An emission filter for regulating andreducing carbon-containing particulates included in an exhaust gas froman internal combustion engine, said emission filter comprising: aheat-resistant filter medium that collects a hydrocarbon compound andthe carbon-containing particulates included in the exhaust gas in adispersive manner to bring the respective particulates and hydrocarboncompound in contact with oxygen included in the exhaust gas, and therebymakes the collected hydrocarbon compound and the collectedcarbon-containing particulates subjected to combustion with the exhaustgas having a filter inflow temperature lower than a combustibletemperature of the carbon-containing particulates without requiring useof catalyzing agents, wherein said heat-resistant filter medium includesmultiple pathways, which connect with one another in a three dimensionalmaimer inside said filter medium and are open to surface of said filtermedium, the multiple pathways included in said heat-resistant filtermedium have a mean inner diameter in a range of 11 μm to 13 μm, thefilter being self-reproducing without heating by a heater.
 27. Anemission filter for regulating and reducing carbon-containingparticulates included in an exhaust gas from an internal combustionengine, said emission filter comprising: a heat-resistant filter mediumthat collects a hydrocarbon compound and the carbon-containingparticulates included in the exhaust gas in a dispersive manner to bringthe respective particulates and hydrocarbon compound in contact withoxygen included in the exhaust gas, and thereby makes the collectedhydrocarbon compound and the collected carbon-containing particulatessubjected to combustion with the exhaust gas having a filter inflowtemperature lower than a combustible temperature of thecarbon-containing particulates without requiring use of catalyzingagents, wherein said heat-resistant filter medium includes multiplepathways, which connect with one another in a three dimensional mannerinside said filter medium and are open to surface of said filter medium,the multiple pathways included in said heat-resistant filter medium havea mean inner diameter in a range of 11 μm to 13 μm, and saidheat-resistant filter medium is a non-woven fabric made ofheat-resistant fibers having a mean fiber diameter in a range of 15 μmto 20 μm, the subject filter being self-reproducing without heating by aheater.